This invention relates generally to color imaging and more particularly to the use of plural exposure and development steps for such purposes.
One method of printing in different colors is to uniformly charge a charge retentive surface and then optically expose the surface to information to be reproduced in one color. This information is rendered visible using marking particles followed by the recharging of the charge retentive surface prior to a second exposure and development. This recharge/expose/and develop process may be repeated to subsequently develop images of different colors in superimposed registration on the surface before the full color image is subsequently transferred to a support substrate. The different colors may be developed on the photoreceptor in an image on image development process, or a highlight color image development process (image next-to image). The images may be formed by using a single exposure device, e.g. ROS, where each subsequent color image is formed in a subsequent pass of the photoreceptor (multiple pass). Alternatively, each different color image may be formed by multiple exposure devices corresponding to each different color image, during a single revolution of the photoreceptor (single pass).
Several issues arise that are unique to the image on image process of creating multi-color images, in the attempt to provide optimum conditions for the development of subsequent color images onto previously developed color images. For example, during a recharge step, it is important to level the voltages among previously toned and untoned areas of the photoreceptor so that subsequent exposure steps (and the development thereof) are effected across a uniformly charged surface. The greater the difference in voltage between those image areas of the photoreceptor previously subjected to a development and recharge step; those image areas subjected to a development step, but not yet subjected to a recharge step; and those bare non-developed, untoned areas of the photoreceptor, the larger will be the difference in the development potential between these areas for the subsequent development of image layers thereon.
Another issue that must be addressed with image on image color formation using a recharge step is the residual charge and the resultant voltage drop that exists across the toner layer of a previously developed area of the photoreceptor. Although it may be possible to achieve voltage uniformity by recharging this previously toned layer to the same voltage level as neighboring bare areas, the associated residual toner voltage (V.sub.t) prevents the effective voltage above any previously developed toned areas from being re-exposed and discharged to the same level as neighboring bare photoreceptor areas which have been exposed and discharged to the actual desired voltage levels. Furthermore, the residual charge associated with previously developed toner images reduces the dielectric and effective development field in the toned areas, affecting the consistency and desired uniformity of the developed mass of subsequent toner images. The problems become increasingly severe as additional color images are subsequently exposed and developed thereon. Color quality is severely threatened by the presence of the toner charge and the resultant voltage drop across the toner layer. The change in voltage due to the toned image can be responsible for color shifts, increased moire, increased color shift sensitivity to image misregistration and motion quality, toner spreading at image edges, and loss in latitude affecting many of the photoreceptor subsystems. Thus, it is ideal to reduce or eliminate the residual toner voltage of any previously developed toned images.
Based on the foregoing, a highly reliable and consistent manner of recharging the photoreceptor to a uniform level and minimizing the residual voltage on previously toned areas is needed so that developability is not threatened when applying subsequent toner images on previous toned image layers.
The following references may be found relevant to the present disclosure.
U.S. Pat. No. 4,791,452 relates to a two-color imaging apparatus wherein a first latent image is formed on a uniformly charged imaging surface and developed with toner particles. The charge retentive surface containing a first developed or toned image, and undeveloped or untoned background areas is then recharged by a scorotron charging device prior to optically exposing the surface to form a second latent electrostatic image thereon. An electrical potential sensor detects the surface potential level of the drum to ensure that a prescribed surface potential level is reached. The recharging step is intended to provide a uniformly charged imaging surface prior to effecting a second exposure.
U.S. Pat. No. 4,819,028 discloses an electrophotographic recording apparatus capable of forming a clear multicolor image including a first visible image of a first color and a second visible image of a second color on a photoconductive drum. The electrophotographic recording apparatus is provided with a conventional charger unit and a second corona charger unit for charging the surface of the photoconductive drum after the first visible image is formed thereon so as to increase the surface potential of the photoconductive drum to prevent the first visible image from being mixed with a second color and also from being scratched off from the surface of the photoconductive drum by a second magnetic brush developing unit.
U.S. Pat. No. 4,761,669 relates to creating two-color images. A first image is formed using the conventional xerographic process. Thus, a charge retentive surface is uniformly charged followed by light exposure to form a latent electrostatic image on the surface. The latent image is then developed. A corona generator device is utilized to erase the latent electrostatic image and increase the net charge of the first developed image to tack it to the surface electrostatically. This patent proposes the use of an erase lamp, if necessary, to help neutralize the first electrostatic image. A second electrostatic image is created using an ion projection device. The ion image is developed using a second developer of a different color.
U.S. Pat. No. 4,033,688 discloses a color copying apparatus which utilizes a light-lens scanning device for creating plural color images. This patent discloses multiple charge/expose/develop steps.
US. Pat. No. 4,833,503 discloses a multi-color printer wherein a a recharging step is employed following the development of a first image. This recharging step, according to the patent is used to enhance uniformity of the photoreceptor potential, i.e. neutralize the potential of the previous image.
U.S. Pat. No. 4,660,059 discloses an ionographic printer. A first ion imaging device forms a first image on the charge retentive surface which is developed using toner particles. The charge pattern forming the developed image is neutralized prior to the formation of a second ion image by a corona generating unit and an erase lamp.
U.S. Pat. No. 5,208,636, discloses a printing system wherein charged area images and discharged area images are created, the former being formed first and the latter being proceeded by a recharging of the imaging surface.
U.S. Pat. No. 5,241,356 discloses a multi-color printer wherein charged area images and discharged area images are created, the former being formed first, followed by an erase step and a recharge step before the latter is formed. An erase lamp is used during the erase step to reduce voltage non-uniformity between toned and untoned areas on a charge retentive surface.
U.S. Pat. No. 5,258,820 discloses a multi-color printer wherein charged area images and discharged area images are created. An erase lamp is used following development of a charged area (CAD), and a pre-recharge corona device is used following development of a discharged area (DAD) and prior to a recharge step, to reduce voltage non-uniformity between toned and untoned images on a charge retentive surface.
Application No. Hei 1-340663, Application date Dec. 29, 1989, Publication date Sep. 4, 1991, assigned to Matsushita Denki Sangyo K.K., discloses a color image forming apparatus wherein a first and second charging device are used before exposure and development of an image, where the potential of the image forming unit is higher after passing the first charging device than after passing the second charging device, to eliminate the contrast potential reduction normally caused by applying color toners onto other color toners, and also to prevent toner spray during the exposure process.
The concurrently filed, copending application for patent entitled "Method and Apparatus for Reducing Transferred Background Toner", Ser. No. 08/346,708, filed on 30 Nov. 1994, by a common assignee as the present application, discloses a recharge step between two image creation steps for conditioning a charge retentive surface pursuant to forming the second of the two images, wherein a corona generating device is used to both charge the toned and untoned areas of the charge retentive surface, and to keep wrong-charge toner developed in the background areas at a charge level distinct from the toner developed in the image areas, so that the wrong-charge background toner does not transfer to a support substrate with the image.
The concurrently filed, copending application for patent entitled "Split Recharge Method and Apparatus for Color Image Formation", Ser. No. 08/347,617, filed on 30 Nov. 1994, by a common assignee as the present application, discloses a recharge step between two image creation steps for recharging a charge retentive surface to a predetermined potential pursuant to forming the second of the two images. A first corona generating device recharges the charge retentive surface to a higher absolute potential than a predetermined potential, and then a second corona generating device recharges the charge retentive surface to the predetermined potential. An electrical charge associated with the first image is substantially neutralized after being recharged by the first and second corona generating device.
A number of commercial printers employ the charge/expose/ develop/recharge imaging process. For example, the Konica 9028, a multi-pass color printer forms a single color image for each pass. Each such pass utilizes a recharge step following development of each color image. The Panasonic FPC1 machine, like the Konica machine is a multi-pass color device. In addition to a recharge step the FPC1 machine employs an AC corona discharge device prior to recharge.